So basically, we tried to apply game dynamics to retaining visitors to websites by offering a reward (ie. discount coupon / premium content, etc.) for meeting various 'objectives' (ie. pages to visit). We call it ScavSite and it just creates a toolbar along the bottom of your website with information about the next objective to meet and the current points accumulated.
We've just gone live over at scavsite.com, and although the original four week goal was to make $20, right now we'd be really happy if a few people found this useful. :) It's been a long journey. We built it in django, which neither of us had used before, and it's been an incredible learning experience.